A few minutes a day is healthy for a betta! It only becomes stressful if you leave the mirror in the tank or in the fishes view for an extended period of time. This is because the reflection in the mirror makes your betta think that another betta is envading their territory. But once the reflection is gone, they feel as though they have successfully defended their turf. However, if there's always another "betta" in their tank in plain sight, they feel the need to hide and thus become stressed. At least, that's what I've read. (: